The present invention relates to a process for the continuous preparation of diaryl carbonates from phosgene and at least one monohydroxy compound (monophenol) in the presence of catalysts, and also the use thereof for preparing polycarbonates. The hydrogen chloride formed in the reaction is converted by thermal oxidation into chlorine, with the chlorine being recirculated to the preparation of phosgene. In particular, the process comprises utilization of the hydrogen chloride formed for the process for preparing diphenyl carbonates (DPC process).
It is known that diaryl carbonates, in particular diphenyl carbonate, can be prepared by phase boundary phosgenation (Schotten-Baumann reaction) of monophenols in an inert solvent in the presence of alkali and a catalyst. Here, the use of solvents and sodium hydroxide is disadvantageous since the aqueous alkali can cause partial hydrolysis of phosgene or chlorocarbonic esters and large amounts of sodium chloride are obtained as by-product and the solvent and catalyst have to be recovered.
For this reason, the preparation of diaryl carbonates and in particular diphenyl carbonate by reaction of monophenols and phosgene without alkali and without use of solvents in the presence of a catalyst in the direct phosgenation process has also been examined and has been described in principle in the literature.

Proposals for processes without solvent using soluble catalysts are described in U.S. Pat. Nos. 2,837,555, 3,234,263 and 2,362,865.
There have also been proposals for using heterogeneous, insoluble catalysts which make the work-up of the reaction mixture substantially easier. Thus, EP 516 355 A2 recommends, in particular, aluminium trifluoride which is applied to supports such as aluminosilicates. However, the synthesis of aluminium fluoride is very complicated and expensive due to the handling of fluorine or hydrofluoric acid.
Furthermore, WO 91/06526 describes metal salts on porous supports as catalysts for the reactions according to the invention. A fully continuous phosgenation of phenol over such catalysts is possible only in the gas phase, but this results in relatively high reaction temperatures and the risk of decomposition of the sensitive chloroformic esters. A phosgenation of phenol using these catalysts in the liquid phase can obviously not be carried out since hot, liquid phenol washes out the active catalyst constituents.
Unsupported catalysts which have the great advantage that the catalyst can be separated off very easily and no impurities remain in the crude reaction product have therefore been proposed. The work-up is substantially simplified thereby.
Thus, processes for preparing diaryl carbonates by phosgenation of monophenols in the presence of heterogeneous catalysts such as activated carbons (EP 483 632), aluminium oxides (EP 635 477), aluminosilicates (EP 635 476), metal oxides (EP 645 364), metalates (EP 691 326), hard materials (EP 722 930) and mixed hydroxides (DE 10 2008 050 828) and also in the presence of homogeneous catalysts such as metal salts (U.S. Pat. No. 634,622), aromatic nitrogen heterocycles (D-A 2 447 348) and organophosphorus compounds (U.S. Pat. No. 5,136,077) both in the liquid phase (EP 757 029, EP 791 574) and also in the gas phase (EP 808 821) have been described.
After synthesis of the diaryl carbonate, the diaryl carbonate is separated off as a mixture with the monophenol used and possibly catalyst or if appropriate in the form of its solution in the organic solvent used in the synthesis, for example chlorobenzene.
To obtain the high-purity diaryl carbonate, a purification by distillation and/or crystallization can be carried out. For example, this is carried out by means of one or more distillation columns connected in series in which any solvent is separated from the diaryl carbonate.
This purification stage or stages can, for example, be carried out continuously in such a way that the temperature at the bottom in the distillation is from 150° C. to 310° C., preferably from 160 to 230° C. The pressure employed for carrying out this distillation is, in particular, from 1 to 1000 mbar, preferably from 5 to 100 mbar.
The diaryl carbonates which have been purified in this way have a particularly high purity (GC>99.95%) and very good transesterification behaviour, so that a polycarbonate can subsequently be prepared in excellent quality therefrom.

The use of the diaryl carbonates for preparing aromatic oligocarbonates/polycarbonates by the melt transesterification process is known in the literature and is described, for example, in the Encyclopedia of Polymer Science, Vol. 10 (1969), Chemistry and Physics of Polycarbonates, Polymer Reviews, H. Schnell, Vol. 9, John Wiley and Sons, Inc. (1964), pp. 50/51 or U.S. Pat. No. 5,340,905.
The hydrogen chloride formed in the preparation of diphenyl carbonate by direct phosgenation of phenol can be utilized, for example, by marketing of the aqueous solution (hydrochloric acid) or by use in syntheses of other chemical products. However, the amounts of hydrogen chloride obtained cannot always be marketed or used for other syntheses in their entirety. In addition, hydrogen chloride can be used for syntheses only when it is appropriately purified beforehand. On the other hand, marketing is usually only economical when the hydrogen chloride or the hydrochloric acid do not have to be transported over long distances.
One of the most common possible uses of the hydrogen chloride obtained is therefore the use as raw material in PVC production, in which ethylene is oxychlorinated by means of hydrogen chloride to form ethylene dichloride. However, this mode of operation is not generally possible since the corresponding production operations are usually not in the direct vicinity of a diaryl carbonate production plant. Disposal of the hydrogen chloride, e.g. after neutralization with alkali, is unattractive from an economical and ecological point of view.
A recycling process for the hydrogen chloride and recirculation of the chlorine to the diphenyl carbonate production process in which hydrogen chloride is obtained is therefore the desired mode of operation.
A review of electrochemical recycling processes is given in the article “Chlorine Regeneration from Anhydrous Hydrogen Chloride” by Dennie Turin Mah, published in “12th International Forum Electrolysis in Chemical Industry—Clean and Efficient Processing Electrochemical Technology for Synthesis, Separation, Recycle and Environmental Improvement, Oct. 11-15, 1998, Sheraton Sand Key, Clearwater Beach, Fla.”.
The recycling of hydrogen chloride by electrochemical oxidation to form chlorine and hydrogen is described in LU 88 569 and EP 1 112 997. A disadvantage is the low current yield and the production of hydrogen which has no use in the polycarbonate production process.
The recycling of hydrogen chloride by thermal reaction with oxygen in the presence of catalysts is also known, as described, for example, in WO 04/014845. A disadvantage here is that the recovery of the phosgenation catalyst pyridine by treatment with alkali leads to a reduced recovery of hydrogen chloride due to the formation of sodium chloride, which has to be disposed of.
Proceeding from the prior art cited above, it was an object of the present invention to provide a diaryl carbonate production process which gives products in high purity and good yield and in which a reduction in environmental pollution or wastewater problems in water treatment plants is achieved by maximized recycling of by-products originating from polycarbonate production. In particular, in the recycling, the conversion of hydrogen chloride into chlorine should be carried out with minimal energy input and therefore in a resource-conserving manner.
It has now been found that hydrogen chloride can be reused particularly advantageously when this hydrogen chloride is converted back into chlorine by thermal oxidation by means of oxygen by the Deacon process and is utilized for preparing phosgene.
The hydrogen chloride obtained in the continuous preparation of diaryl carbonates by reaction of monophenols and phosgene in the presence of catalysts can be passed directly without complicated purification, if appropriate after simple treatment with activated carbon, to a thermal oxidation, with the hydrogen chloride being converted catalytically into chlorine and water and the chlorine being recirculated to the preparation of phosgene.